1. Field of the Invention
The present invention relates to a torque distribution control device for controlling the distribution of drive torque to sub-drive wheels of a four-wheel drive vehicle.
2. Discussion of the Related Art
A torque distribution control device for a four-wheel drive vehicle has been known as described in Japanese unexamined, published patent application No. 2001-71781, wherein a torque transmission clutch is provided for distributing the torque transmitted from an engine to prime drive wheels, to sub-drive wheels in dependence on the rotational speed difference between the prime drive wheels and the sub-drive wheels. In the known control device, when the acceleration of the vehicle exceeds a predetermined value, an acceleration mode map which has been set to enlarge the rate of the increase in the torque distributed to the sub-drive wheels to the increase in the rotational speed difference between the prime drive wheels and the sub-drive wheels is selected, so that a lager torque relative to the rotational speed difference is distributed to the sub-drive wheels. When the acceleration of the vehicle remains below the predetermined value, on the contrary, a tight mode map which has been set to diminish the rate of the increase in the torque distributed to the sub-drive wheels to the increase in the rotational speed difference is selected, so that a small torque relative to th rotational speed difference is distributed to the sub-drive wheels.
According to the control method in the known torque distribution control device, the acceleration and the starting of the vehicle on a road with a small coefficient of friction can be performed stably with the sub-drive wheels being given a large torque, and a slow-speed turn at a tight-corner can be performed with the sub-drive wheels being given a small torque, thereby avoiding the occurrence of a so-called “tight-corner braking phenomenon”.
However, when a turn at a corner is made at a medium speed with the acceleration smaller than the predetermined value, it cannot be attained only by judging whether the acceleration of the vehicle is less than the predetermined value or not, to distribute to the sub-drive wheels the torque sufficient to enhance safety. As noted above, in the known method, the traveling state cannot be grasped sufficiently, so that it cannot be done to distribute to the sub-drive wheels the torque which directly reflects the vehicle traveling state or the driver's will.
Further, because the magnitude of the torque distributed adequately to the sub-drive wheels is varied in dependence on the states of the vehicle acceleration and the throttle opening degree, it cannot be done only by judging whether the acceleration exceeds the predetermined value or not, to distribute the adequate torque which reflects the drivers will satisfactorily. For example, when the acceleration is smaller than the predetermined value, it cannot be distinguished whether such is due to the fact that the load is large when the acceleration is to be made or due to the fact that the throttle opening degree is made small to enter a constant speed traveling after the acceleration to reach a target vehicle speed zone. Consequently, it could not occasionally be realized to reflect the driver's will or the traveling state, namely to increase the torque distributed to the sub-drive wheels when the acceleration is to be made, or to decrease the torque distributed to the sub-drive wheels when a constant speed traveling is to be made upon completion of the acceleration.